13" mid-2012 MBP, upgraded to 16GB, but only one port recognized as 8GB

Lajafowokan wrote:

I just upgraded my 13" mid-2012 MBP from the stock 4GB to 16GB, using 1 Crucial 16GB modules. On checking the system profiler, I only see one 8GB module recognized; instead of 16GB, the second port shows as empty, as I can't recall leaving the original 4GB in the port.

The MBP is quite fast, but appears I'm not getting the desired speed of 16GB.

Please advise

You can try re-seating the RAM if one of the banks is not recognized.

MacBook Pro: How to remove or install memory - Apple Support

you are testing for both bad RAM or bad bank—

one stick at a time in each bank, If the RAM sticks prove to work individual—and banks work individually.

Other known issues are: 

worn clips

or RAM banks slightly distorted/out of flat (use shims)

Best to have matched pairs, however you can run on just one bank if necessary.

or replace bad RAM—again matched pairs (manufacturer/ same size ie 8 + 8/ exact specs)

Best to buy RAM from reliable source with the exact specs.

Usually a lifetime guarantee on RAM from third party source like:

https://eshop.macsales.com/shop/apple/memory/